1. Field of the Invention
The present invention relates to a semiconductor optical modulator and a method of fabricating the same, and more particularly, a semiconductor optical modulator in which a resistor for impedance matching is integrated in a device using a semiconductor doped layer and a method of fabricating the same.
2. Discussion of Related Art
Recently noticed light sources for optical communication include a 1.55 μm wavelength distributed feedback (DFB) laser, a LiNbO3 external optical modulator, an electroabsorption optical modulator, etc.
The directly modulated DFB laser that has been used for a conventional optical communication system as the light source of a high speed modulation optical communication system has restriction on the transmission distance in a long distance optical communication system having a transmission speed of no less than several Gbps due to change in a refractive index due to the implantation of carrier and increase in the line-width of modulated light caused by the change in the refractive index, that is, a chirping phenomenon. Also, due to the restriction of the modulation speed, the directly modulated DFB laser cannot be used for a long distance optical communication system having a transmission speed of 40 Gbps.
In order to solve the above-described problem, instead of the direct modulation method of the DFB laser that is a light source, a method of performing modulation using an external modulator is used. According to the light source that uses the external modulator method, the modulation line width of output light is very narrow such that it is possible to perform ultra high speed long distance transmission by a conventional dispersive optical fiber and to realize a modulation speed of 40 Gbps.
In the external modulator, the electroabsorption modulator is a device for modulating an optical signal using a phenomenon in which an absorption coefficient changes in accordance with an electric field applied to a semiconductor. Since the electroabsorption modulator is small and manufactured by a standardized semiconductor fabricating process, mass production can be performed at low price. According to the fabricated electroabsorption optical modulator device, the input reflection coefficient of radio frequency (RF) should be reduced in a desired frequency band for a high speed modulation characteristic. If possible, the input reflection coefficient should be reduced to less than −10 dB.
Therefore, as illustrated in FIG. 1, according to the conventional common semiconductor optical modulator device, an optical modulator device 13 is wire bonded 12 to a dielectric substrate 11 in which a resistor 14 for impedance matching is formed. However, according to the structure of the conventional semiconductor optical modulator device 13, the cost related to an additionally required package process occurs. To be specific, the impedance matching resistor 14 is fabricated by a thin film process in which a mask is additionally formed on the dielectric substrate on which a metal electrode is commonly formed, which leads to an increase in the cost. Also, an additional package process of connecting the fabricated dielectric substrate to the optical modulator device by die bonding and wire bonding is required. According to the structure of the conventional semiconductor optical modulator device, it is difficult to perform the wire bonding process for the ultra high speed modulation characteristic of 40 Gbps. Also, since the inductance component of Au wire of a thickness of 1 mil has a value of about 1 nH/mm and wire of a length of 100 μm also has a significant impedance value of about 25Ω in a frequency region of 40 GHz, it is difficult to perform electric connection with a low inductance component and to perform a process with reproducibility.
On the other hand, as illustrated in FIG. 2, there is a conventional method of forming a matching resistor 21 between optical modulator device 22 electrodes using a metal deposition process such as NiCr. However, in order to fabricate an optical modulator device using the above-described conventional method, a metal deposition process for a NiCr process and a photo transfer process using a photo mask are additionally required.
According to the above-described conventional technology, it is difficult to reduce package process related cost and to secure an ultra high frequency characteristic with reproducibility.